The present invention relates to a process and an installation for protecting a solid metal against oxidation during a rolling operation.
Some metallurgical processes carried out on heated solid metals and in contact with oxygen of the air result in the formation of layers of oxide which adversely affect the quality of the surface of the finished product. This problem is well known in the case of heat treatment in which are employed controlled reducing atmospheres formed by a carrier gas, such as nitrogen, argon or a mixture of these gases with hydrogen to which may be added an active gas (hydrocarbon). These atmospheres permit the control of the surface condition of the metal or the modification of the surface composition of the metal, for example by carburizing it, but in any case they avoid the oxidation.
In the case of heat treatments, the problem of the oxidation of solid metals subjected to these treatments is solved owing to the confinement of the atmosphere, due to the furnace. However, this confinement does not exist in the case of other metallurgical processes such as rolling.
In the particular case of rolling, the problems of oxidation and the descaling they require in the rolling line are very great. Indeed, the following operations appear in succession in such a rolling line:
a. Between the furnace and the roughing train, the descaling is carried out mechanically and is based on the difference of plasticity between the surface oxide formed and the metal. PA0 b. In the course of the roughing, the descaling is carried out by means of interposed descaling cages operating with water under high pressure, on the order of 100 to 150 bars. PA0 c. On the downstream side of the roughing stand, a new hydraulic descaling is carried out before the finishing stand, so as to eliminate the scale formed on the waiting table. PA0 d. Finally, a pickling with acid is effected following the cooler so as to eliminate surface defects.
These multiple operations result in a number of drawbacks. For example, there is a loss of metal by oxidation, and additional consumption of power takes place owing to the descaling with water under pressure. Furthemore, there is a substantial drop in temperature of the metal owing to the descalling with water, and it is necessary to dispose of the iron sulphates and chlorides resulting from the acid pickling.
Confronted with such a situation, there is at present observed a probable evolution of the rolling operation. For example, in the case of hot rolling, there is envisaged the use of the planetary rolling mill which permits great reductions in thickness in a single pass. This technique requires a very low speed of introduction of the metal into the planetary rolling mill so as to avoid reaching excessively high speeds at the output end. However, this favours a re-oxidation of the metal before it enters the planetary rolling mill. Moreover, a mechanical descaling by shot-blasting is not used since it would give rise to problems of working conditions, placement of the equipment and encrusting in the metal.